Erysipelothrix rhusiopathiae is a gram-positive, rod-shaped, nonmotile, nonsporulating, facultatively anaerobic bacterium that is widely distributed in nature and is a commensal of the oropharyngeal and intestinal mucosa of various mammals, fish, and birds. Opportunistic entry of the organism into deeper tissues results in disease in a variety of hosts, including humans. Human infection is frequently associated with occupational exposure to the microorganism and may result in a variety of clinical conditions, including skin lesions and acute or subacute endocarditis. Swine, lamb, and turkey erysipelas cause significant economic losses and can result in exposure of humans to the agent. Clinical signs of disease in swine range from subacute urticarial lesions to chronic arthritis and/or endocarditis to acute septicemia and death.
Because E. rhusiopathiae is a facultative intracellular pathogen that survives well within mouse peritoneal macrophages and pig polymorphonuclear neutrophils, protection of swine from disease is thought to rely on eliciting both humoral and cell-mediated immunity. Commercial vaccines have been shown to elicit cross-protective immunity to challenge with serotypes 1 and 2. Although both cross-reactive and serotype-specific antibodies are elicited in swine by exposure to the organism, not all elicited antibodies are protective in nature, and a serological response to whole-cell antigen does not correlate with immunity following challenge-exposure to the agent.
There are a number of USDA-licensed whole-cell bacterins (killed vaccines) and modified-live virus vaccines useful in the control of swine erysipelas. However, it has been suggested that vaccination of swine may lead to an increase in arthritic lesions, possibly as a result of hypersensitization of the animal to subsequent exposure to virulent E. rhusiopathiae. Previously reported monoclonal antibodies have been shown to prevent the development of polyarthritis induced by experimental infection with E. rhusiopathiae in rats. There was evidence that prevention of disease by the previously reported monoclonal antibody was not due to classical passive immunization, but rather to monoclonal antibody activation of host immune responses. The previously reported monoclonal antibody was not protective when given with or after the challenge-exposure.
Whole-cell and detergent-solubilized surface protein extracts of E. rhusiopathiae have been shown to contain a number of immunogens; however, most immunogens have not been examined with respect to the ability to induce a protective response or a hypersensitizing response. Of particular interest is a 64-66 kDa band complex that has been shown to contain 1-4 bands (depending on experimental conditions) that are immunogenic in swine. A gene encoding a protective 64-66 kDa antigen complex was cloned and expressed in Escherichia coli. The expressed gene was shown to result in 43 and 64-66 kDa components. The 43 kDa polypeptide is thought to be a processed or degraded form of the 64-66 kDa protein and has been shown to react with antisera that recognizes the 64-66 kDa protein. The recombinant E. coli elicited a protective immunologic response in mice. A serological response in swine to a 65 kDa antigen was shown to correlate with a protective immune response. In addition, a 65 kDa antigen, which is highly expressed by E. rhusiopathiae, has been shown to share significant homology and antigenic specificity with the E. coli DnaK gene product, a 70 kDa heat shock protein (Hsp70). However, it is not known if the 65 kDa protective immunogen is the same protein as that shown to have homology with the Hsp70.
Because the 65 kDa protein has been shown to be an immunodominant detergent-soluble antigen, it has been suggested that if the protective immunogen(s) are different from the sensitizing immunogen(s), a subunit vaccine composed of only protective immunogens, such as the 65 kDa protein, would provide a safer vaccine. Ideally, a subunit vaccine would protect against both the acute and chronic forms of the disease without detrimental hypersensitizing effects such as arthritis. A subunit vaccine might also provide a means for a diagnostic serologic assay for differentiating vaccinates from those animals exposed to field strains of E. rhusiopathiae.
Heat shock proteins and other bacterial stress proteins have been associated with protective host immune responses and have been identified as immunodominant B-cell antigens of diverse bacterial pathogens. It has been postulated that these abundant stress proteins may have immunoprophylactic potential for a broad spectrum of pathogens. The DnaK protein is a member of a highly conserved family of proteins found in all living cells that function in the translocation of proteins across cell membranes, most likely by acting as molecular chaperones. These proteins are abundantly produced by bacterial cells during an infective process. For these reasons, it was felt likely that a hybridoma secreting monoclonal antibodies (MAb) to the E. rhusiopathiae 64-66 kDa polypeptide would be useful for potency testing of erysipelas bacterins.